Lewin's NCF Experiment and Lecture

[tinman]

Ok,so following your instructions Poynt, i ran some lead wires along the resistor loop wire,and away from the solenoid--as seen in pic below.
Happy to report no voltage at all detected across the wire between the resistors after 10 go's.

So i applied the same to the resistors,and also happy to report that i have a total voltage across the resistors that is very close to that of the 980mV measured with the scope probe and ground loop around the solenoid.

So all things are as they should be now
Now-one problem . Dose this not confirm Lewins experiment?.


Brad

[poynt99]

Ok,so following your instructions Poynt, i ran some lead wires along the resistor loop wire,and away from the solenoid--as seen in pic below.
Happy to report no voltage at all detected across the wire between the resistors after 10 go's.

So i applied the same to the resistors,and also happy to report that i have a total voltage across the resistors that is very close to that of the 980mV measured with the scope probe and ground loop around the solenoid.

So all things are as they should be now
Now-one problem . Dose this not confirm Lewins experiment?.


Brad

Good stuff Brad.

Yes, you have replicated all the results and shown the effects of Lewin's experiment. As such, it would appear that Lewin's emphatic push to denounce Kirchhoff as not holding in such a case seems to be true, correct? (One can easily come to this conclusion because when summing the voltages measured around the loop, we come to the same voltage as the calibrated voltage, and it is not zero as Kirchhoff would prescribe).

Well, Lewin's experiment only demonstrated half the picture, and attempted to mislead the audience into thinking he was measuring potential differences in the circuit. The fact is that this is not entirely true, and the experiment as demonstrated only captures one perspective of reality. He is in fact measuring the E field, and it so happens that the E field in both perspectives overlaps when it comes to the values measured across the resistors. It does not however overlap when it comes to measuring the wire segments.

How then do we examine the experiment from the second perspective (the "real" one Lewin conveniently left out of his demonstration)? Simply make a twin lead cable long enough that it can run from your scope probe, to the ceiling (screw-hook), then drop down right above the solenoid so it can clip onto the various measurement points. This effectively decouples the measurement device from the experiment and permits us to measure the "real" potential differences across all of the components, i.e. the resistors and wire segments in between.

Redo the measurements using this "perspective", and compare your results. Be sure to measure across points D and A again too (i.e. across the middle of the wire segments as per Lewin's diagram), but first try to make a prediction of what voltage (if any) you will measure there.

[wattsup]

Redo the measurements using this "perspective", and compare your results. Be sure to measure across points D and A again too (i.e. across the middle of the wire segments as per Lewin's diagram), but first try to make a prediction of what voltage (if any) you will measure there.

Zero, because the potential will be equal from both sides.

wattsup

[poynt99]

Try also to predict what will be measured (if anything) across the wire segments.

[Magluvin]

Zero, because the potential will be equal from both sides.

wattsup

Should be in normal cases.  But this is sort of a current source event considering the 2 different value resistors are in a current loop and the wires are the current induced parts.

We once did an audio system in a mercedes. We had an engine/alternator wine in the tweeters we just couldnt figure out. Checked all grounds, rca cables. Repl amps, head unit, etc.   Come to find that the passive crossover network box was located near a car wire harness. It was inducing the coils in the crossover. Not that it was the same thing you guys were figuring out, but.....

Nice work on working out the scope lead problems.

Mags